The present invention relates to a method for forming a photoresist mask utilized in various fabrication processes of semiconductor devices, thin-film devices such as thin-film magnetic heads, thick-film devices and other devices.
In photolithographic processes or sputtering processes executed during fabrication of such devices, undercut photoresist masks (photoresist masks with undercuts in their lower portions) may be sometimes used so as to obtain patterned layers with desired tapered sidewall faces without burr.
There are two known approaches for forming the undercut photoresist mask. One is an image reversal photoresist process and the other is a double photoresist layers process.
The former process, which is described in for example Japanese patent unexamined publication No.4-46346 and U.S. Pat. No. 5,658,469, uses a novolak type positive tone photoresist capable of image reversal, and renders the positive photoresist to negative by post exposure bake and flood expose processes.
FIGS. 1a-1e are sectional views illustrating processes of forming the photoresist mask by using this image reversal photoresist.
First, as shown in FIG. 1a, a photoresist layer 11 is coated on a substrate 10. Then, as shown in FIG. 1b, a pattern of a photoresist mask to be obtained is exposed by a relatively little amount of exposure via a photomask 12. Reference numeral 13 in the figure represents the exposed pattern or area of the photoresist layer 11. Then, as shown in FIG. 1c, the post exposure bake process is performed to convert the photoresist within the exposed region 13 into insoluble. Thereafter, as shown in FIG. 1d, the flood expose process is performed to expose entire region of the photoresist layer 11. Then, by developing, a part of a lower portion 14 of the photoresist layer 11 in the exposed region 13 is removed to provide an undercut photoresist sidewall as shown in FIG. 1e. 
However, according to this approach, since height and width of the undercut profile may vary depending upon a penetrating degree of a developer or solvent into the photoresist material under the area 13, it is very difficult to always keep a desired shape of the undercut profile. Thus, no volume production of the devices can be expected. Furthermore, this approach cannot be applied to the lift-off process of a thick-film layer because of limitation of the height of the fabricated undercut profile.
The latter process, namely the double photoresist layers process, is well known. According to this process, first, a lower photoresist layer is coated on a substrate, and then baked. Next, an upper photoresist layer made of material which will never mix with that of the lower photoresist layer is coated on the lower photoresist layer. Then, a photoresist pattern is transferred from a photomask to these two layers and thereafter development is performed. Since etch rates of these two layers with respect to the developer differ with each other, namely the lower layer is more soluble than the upper layer, a desired undercut profile of the photoresist mask can be obtained.
This double photoresist layers process needs however two coating steps for the lower and upper photoresist layers and a baking step between the coating steps causing the number of processing steps to increase and also the fabricating process to complicate. Furthermore, since removal of a part of the lower photoresist layer for generating an undercut is difficult, the removing step may be sometimes necessary to execute twice causing the number of processing steps to further increase. In addition, since the shape of the undercut is controlled based upon the differing etch rates between the lower and upper photoresist layers, simultaneous control of the height and width of the undercut is very difficult. As well as the aforementioned approach, this approach cannot be adopted to the lift-off process of a thick-film layer because of limitation of the height of the fabricated undercut profile.
It is therefore an object of the present invention to provide a method for forming a photoresist mask, whereby various undercut profiles can be easily produced in accordance with requirements from fabricating process of devices.
According to the present invention, a method for forming a photoresist mask includes a step of forming a photoresist layer, exposure depth of the layer being controllable depending upon exposure amount, and a step of performing multiple exposes of the formed photoresist layer at different exposure amounts via a plurality of photomasks with different opening patterns, respectively.
Also, according to the present invention, a method for forming a photoresist mask with a lower portion and an upper portion which is wider than the lower portion, includes a step of forming a photoresist layer, exposure depth of the layer being controllable depending upon exposure amount, and a step of performing multiple exposes of the formed photoresist layer at different exposure amounts via a first photomask with an opening pattern corresponding to a pattern of the lower portion and via a second photomask with an opening pattern corresponding to a pattern of the upper portion, respectively.
Since multiple exposes of a photoresist layer of which exposure depth is controllable depending upon exposure amount are executed at different exposure amounts via a plurality of photomasks with different opening patterns, respectively, the exposure depth corresponding to each photomask pattern can be appropriately controlled. Thus, a three-dimensionally shaped photoresist mask with a re-entrant profile such as an arched profile or an undercut profile can be precisely and easily formed.
It is preferred that the exposure amount at the expose using the first photomask is larger than that at the expose using the second photomask.
It is also preferred that the forming step is a step of forming a photoresist layer made of a positive tone photoresist material capable of image reversal, and that the performing step is a step of performing multiple exposes of the formed photoresist layer before image reversal processing.
More preferably, the photoresist material is a novolak type positive tone photoresist material.
It is preferred that the method further includes a step of executing post exposure bake processing for image reversal of the photoresist layer, a step of executing flood expose processing of the photoresist layer, and then a step of developing the photoresist layer.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.